Amount vs. Number
Amount is used in reference to mass nouns
Number is used in reference to count nouns
A noun denoting something which cannot be counted (e.g. a
substance or quality), in English usually a noun which
- lacks a plural in ordinary usage and
- is not used with the indefinite article,
e.g. We say:
- an amount of furniture (Two furnitures? No! a furniture? No!)
- an amount of happiness. (Two happinesses? No! a happiness? No!)
A noun that can form a plural and, in the singular, can be used
with the indefinite article
e.g. We say:
- a number of books (Two books? Sure! a book? Sure!)
- a number of chairs (Two chairs? Sure! a chair? Sure!)
So let's put the definitions and rule to the test with zombies and purchases
- Since zombies is the plural of zombie and it would be quite natural
to say a zombie, we say: A number of zombies.
- Since purchases is the plural of purchase and it would be quite natural to say a purchase, we say: A number of purchases.
Both zombie and purchase are count nouns, and we use number in in reference to count nouns.
Easy mneumonic "count the number of x"
An exception? Not really!
There is a second definition for Mass Noun
1.1 A noun denoting something which normally cannot be counted but which may be countable when it refers to different units or types
In the sense of I bought a pound of coffee We say:
- an amount of coffee (Two coffees? Not in this sense! a coffee? Not in this sense!)
SO here is the confusion!
In the sense of I ordered two coffees, we say:
The mass noun coffee functions as a count noun when it refers to specific:
- units like cups of coffee (two coffees is two cups of coffee)
- types like Columbian coffee or Kenyan coffee (two coffees is two kinds of coffee)
Plural or Singular?
The expression "an amount of..."
will always be singular, because the mass noun it refers to will always be singular. An amount of... denotes a quantity without specific number.
- An amount of furniture does not consider the number of pieces of
- An amount of flour does not consider the number of grains, cups,
pounds or tons of flour.
The expression "a number of..."
SHOULD always be plural, because the count noun is assumed to be plural. If it were singular, you would not use the construction "a number of...". The indefinite article simply denotes that the number is indefinite: it could be 2, 200, 202 or 2002.
- "A significant number of zombies were detected in your city."
We detected more than one zombie, but we don't know exactly how many,
so we say a number of zombies were detected.
In that sentence zombies is acting as the subject and "a number of" is the determiner for zombies, modifying zombies as an adjective (in place of the unspecified number.) The second example works the same way.
- "A significant number of purchases were detected..." We detected more than one purchase, but we don't know exactly how many, so we say a number of purchases were detected.
So why does the "wrong one" sound so right?
"The number of..." will always be singular because the definite number is singular and is functioning as the subject of the sentence.
- "The significant number of zombies being detected in your city
- "The significant number of purchase being
detected...impacts the economy."
The untrained native speaker overlooks the subtle distinction between two constructions: a number of... and the number of... Specifically, the grammatical notion of the determiner is pretty obscure to most folks. It sounds close enough, and before you know it, half the world is saying aint, and there it is--in the dictionary! We don't need perfect grammar to communicate, but the better our grammar, the clearer our communication.